Flip chip bonding method using electrically conductive polymer bumps

ABSTRACT

A method is presented for interconnecting bond pads of a flip chip with bond pads of a substrate by an electrically conductive polymer. A first organic protective layer is selectively formed over a surface of a flip chip to thereby leave exposed bond pads on the flip chip. An electrically conductive polymerizable precursor is disposed on the bond pads extending to a level beyond the organic protective layer. The first and second layers are polymerized to form electrically conductive bumps. A second organic protective layer is then formed on the surface of the substrate around the substrate bond pads, leaving the substrate bond pads exposed. An electrically conductive adhesive is then disposed on the substrate bond pads. The bumps are aligned with bond pads of a substrate and then contacted to those bond pads. The adhesive is polymerized to form electrical interconnections between the flip chip bond pads and the substrate bond pads.

RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.07/452,191, now U.S. Pat. No. 5,074,947, and filed Dec. 18, 1989.

BACKGROUND OF THE INVENTION

Integrated circuits have had almost universal application tocommunication and military technologies for several years. Of increasingimportance has been development of microcircuit wafers and methods forinterconnection of the circuits by automated equipment. A primarylimitation to application of microcircuit technology has been costefficiency and reliability of interconnection of integrated circuits onchips because of the small size of the chips, which often requirehundreds of connections to be made within each circuit.

One method of circuit interconnection is called flip chip bonding. Flipchip bonding can offer a shorter signal path and, therefore, more rapidcommunication between circuits than can other methods, such as tapeautomated bonding (TAB) or conventional wire bonding, because bond padson flip chips are not restricted to the periphery of the chip, butrather are usually located at one face of the chip opposite a substrate.In one method of flip chip bonding, a chip or die is formed with therequisite integrated circuit and interconnect wiring required forinterconnecting the circuit with other chip circuits on a circuit board,such as a separate printed circuit board or substrate. Bond pads arelocated at points of interconnection. Bumps are formed by plating ofseveral layers of metals on the bond pads of the flip chip. Followingdeposition, the chip is heated to reflow the metals, thus causingsurface tension of the deposit to form hemispherical solder "bumps." Theflip chip is subsequently severed from the wafer of which it was a partand "flipped" for alignment with the bond pads of a substrate. Thesebumps are then contacted with the bond pads of the substrate anduniformly heated to simultaneously form interconnects between alignedbond pads of the flip chip and the substrate.

Use of metals to interconnect bond pads of flip chips and substrates hasrequired, however, that passivation of the flip chip be accomplished byuse of a metal barrier such as titanium (Ti), tungsten (W) or siliconnitride (Si₃ N₄). Both the metal, as a passivation (or barrier)material, and ceramic, as a substrate material, are generallynecessitated to allow sufficient heating to enable reflow of the solderbumps for interconnection between the flip chip and the substratewithout consequential damage to either.

Fabrication of circuits using bumped flip chips have also been limitedby the inability to visually inspect interconnections between the flipchip and the substrate. Further, the yield of finished mounted circuitscan be detrimentally affected by failure of interconnects caused by thedifference between the coefficients of thermal expansion of the variousmaterials comprising the flip chip, the passivation layer, the solderbumps and the substrate. Also, melting of the solder bumps creates anelectrically conductive flux as an undesirable byproduct which generallymust be removed from between the substrate and the flip chip to allowproper operation of the finished circuit.

Problems of heat stress during fabrication have been addressed byvarious methods, such as by rapid application of heat to a bumped flipchip and rapid conduction of heat from the solder interconnects in orderto minimize damage to flip chips, substrates and interconnections due tointernal stresses caused by thermal expansion and contraction. However,this method is very expensive.

Therefore, a need exists for a method of interconnecting flip chips tosubstrates which is fast, cost-effective and reliable, so that theadvantages of flip chips over other types of microcircuit wafers can beexploited more fully. Also, there is a demand for a simplified method ofconnecting flip chips to substrates which eliminates the need forelaborate plating procedures. Further, a method which enables greaterflexibility of passivation and choice of substrate is also desirable.These improvements could promote cost efficiency and broaden theapplications for which microcircuits are suitable.

SUMMARY OF THE INVENTION

The present invention relates to a bumped flip chip technology and amethod for interconnecting the bond pads of a bumped flip chip to thebond pads of a substrate. In accordance with the present invention, afirst organic protective layer is selectively formed over the surface ofa flip chip, leaving the flip chip bond pads exposed. An electricallyconductive polymerizable precursor is disposed at the bond pads of theflip chips to form "bumps" which extend beyond the organic protectivelayer. Alternatively, the electrically conductive polymerizableprecursor can be formed in two layers at each bond pad, the two layerstogether forming the bumps. The two layers can be polymerized to form anelectrically conductive bump before connecting the bump with bond padsof the substrate.

A second organic protective layer is selectively formed over a substratearound bond pads of the substrate, leaving the substrate bond padsexposed. An adhesive is then applied to the substrate bond pads toprovide "wet," or electrically conductive, connections between the bumpsand substrate which are subsequently polymerized. An electricallyconductive polymer is thereby selectively formed between the bond padsof the flip chip and the bond pads of the substrate. Alternatively, thebumps can be polymerized after connecting the bumps of the flip chip tothe bond pads of the substrate.

The second organic protective layer extends beyond the level of thesubstrate bond pads and facilitates positioning between the bumps at theflip chip bond pad and the substrate bond pads by forming recessedportions of the substrate at the substrate bond pads. Electricalinterconnections between bond pads of flip chips and bond pads ofsubstrates are obtained by formation of a electrically conductivepolymerizable precursor at the bond pads of a flip chip. Polymerizationof the bumps can be achieved under milder thermal conditions than arerequired to reflow solder. Thus, reliability problems, caused by rapidheating and by large discrepancies of coefficients of thermal expansionof component materials in the flip chip, passivation layer, bumps andsubstrates, can be substantially reduced. Further, because thepolymerization conditions are less harsh than required for reflow ofsolder bumps, the need for metal passivation of the flip chip iseliminated and a wider variety of substrate types is enabled. Also,complicated and time-consuming vapor deposition and electroplatingtechniques for depositing solder bumps are eliminated. In addition,polymer interconnects are fluxless, thus eliminating difficult problemswith removal of electrically conductive flux between flip chips andsubstrates. The organic protective layers can also have a low dielectricconstant, thereby acting as a passivation layer and enabling closeproximity of the flip chip to the substrate and consequent shortenedcircuit paths in the finished circuit.

The above features and other details of the invention, either as stepsof the invention or as combinations of parts of the invention, will nowbe more particularly described with reference to the accompanyingdrawings and pointed out in the claims. It will be understood that theparticular embodiments of the invention are shown by way of illustrationand not as a limitation of the invention. The principle features of theinvention may be employed in various embodiments without departing fromthe scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of one embodiment of the present invention afterselective formation of a first organic protective layer over the surfaceof a flip chip.

FIG. 2 is a section view of the embodiment of FIG. 1 taken along linesII--II.

FIG. 3 is a section view of a flip chip which has been passivated with asilicon nitride or oxide layer, over which layer a first organicprotective layer has been formed.

FIG. 4 is a section view of the embodiment of FIG. 1 after formation ofthe first electrically conductive polymerizable precursor layer on theflip chip bond pads.

FIG. 5 is a section view of the embodiment of FIG. 1 after formation ofa second electrically conductive polymerizable precursor layer on thefirst layer to thereby form bumps.

FIG. 6 is a plan view of a substrate suitable for use with the presentinvention.

FIG. 7 is a section view of the embodiment of FIG. 6 taken along lineVII--VII after a second organic protective layer has been selectivelyformed on the substrate.

FIG. 8 is a section view of the embodiment shown in FIG. 7 after anadhesive layer has been disposed on the substrate bond pads.

FIG. 9 is a section view of the embodiment of FIG. 1 and of thesubstrate shown in FIG. 8 after aligning the bumps of the flip chip withsubstrate bond pads.

FIG. 10 is a section view of the embodiment shown in FIG. 9 aftercontact of the bumps of the flip chip with the substrate bond pads.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the present invention, shown in FIG. 1, asimplified illustrative version of a flip chip 10 is shown. It consistsof bond pads 12,14 on upper planar surface 16 of flip chip die 11. Die11 is formed of silicon, gallium arsenide, germanium or some otherconventional semiconductor material. As can be seen in FIG. 2, a firstorganic protective layer 18 is formed over circuits 15 (connected to thebond pads) and surface 16 of flip chip 10 by screen printing,stenciling, spin-etching or by other methods of monomer or polymerdeposition. Alternatively, flip chip 10 also can be passivated withsilicon nitride or an oxide layer 19 before formation of first organicprotective layer 18, as is shown in FIG. 3. The organic protective layeris preferably a dielectric polymer. An example of an organic materialsuitable for application in the present invention is "Epo-Tek 600®"polyimide, manufactured by Epoxy Technology, Inc. Bond pads 12,14 arecovered during deposition of first organic protective layer 18 and arethen left exposed following deposition, as shown in FIG. 2. Firstorganic protective layer 18 is preferably polymerized by application ofheat or other conventional means prior to formation of layers 20,22 onbond pads 12,14, shown in FIG. 4. First organic protective layer 18passivates and thereby insulates and protects the underlying surface 16of flip chip 10.

As shown in FIG. 4, first layers 20,22 of an electrically conductivepolymerizable precursor are selectively formed on bond pads 12,14.Electrically conductive polymerizable precursor, as that term is usedherein, can include a thermoset polymer, a B-stage polymer, athermoplastic polymer, or any monomer or polymer which, uponpolymerization or upon further polymerization, is electricallyconductive or which can support an electrically conductive material. Theelectrically conductive polymerizable precursor can be gold-filled,silver-filled, or filled with some other electrically conductivematerial. The first organic protective layer 18 acts as a templatedefining areas for deposition of first layers 20,22 of monomer on flipchip 10. In a preferred embodiment of the present invention, theunpolymerized organic protective layer has a high thixotropy forretaining a pattern on surface 16. The flip chip 10 can thus bemanipulated more conveniently during subsequent deposition ofelectrically conductive polymerizable precursor onto bond pads 12,14.First layers 20,22 are substantially flush with polyimide layer 18.Second layers 24,26 of electrically conductive polymerizable precursor,such as are used to form first layers 20,22, are formed on first layers20,22, as shown in FIG. 5. First layers 20,22 and second layers 24,26are then polymerized by a suitable method, such as is known in the art,to form electrically conductive bumps 28,30 on flip chip 10.Alternatively, if the electrically conductive polymerizable precursor isa thermoset, the first layers 20,22 can be polymerized before formationof the second layers 24,26.

As shown in FIG. 6, circuit 33 on substrate 36 is connected with bondpads 32,34. Substrates which are suitable for use with the presentinvention include materials such as ceramic, silicon, porcelain,conventional printed circuit board materials, or other conventionalsubstrates suitable for forming electrical circuits. A second organicprotective layer 37 is selectively formed on substrate 36 around bondpads 32,34 to a level extending beyond the level of substrate bond pads32,34, as illustrated in FIG. 7. Suitable materials for forming secondorganic protective layer 37 and suitable methods of selectively formingsecond organic protective layer 37 are the same as for first organicprotective layer 18.

As seen in FIG. 8, adhesive layers 38,40 are formed on substrate bondpads 32,24 before bumps 28,30 are contacted to substrate bond pads32,34. Examples of adhesives which can be used include thermosets,thermoplastics and polymer thick film. Adhesive layers 38, 40 are formedon substrate bond pads 32, 34 by screen printing, stenciling, or by someother conventional method.

As shown in FIG. 9, bumps 28,30 are located on flip chip is in aposition which is aligned with the known position of bond pads 32,34 onsubstrate 36. As shown in FIG. 10, bond pads 32,34 are then brought intocontact with bumps 28,30. Adhesive layers 38,40 are then polymerized bya suitable method, such as by heating, to form electrically conductiveinterconnections between substrate bond pads 32,34 and bumps 28,30,thereby forming electrical interconnections between flip chip bond pads12,14 and substrate bond pads 32,34.

The electrically conductive polymerizable precursor used to form firstlayers 20,22 and second layers 24,26 of bumps 28,30 can be a B-stagepolymer. Examples of suitable B-stage polymers include thermosets andthermoplastics. Solvents within the B-stage polymer can be substantiallyevaporated from the electrically conductive polymerizable precursorcomprising bumps 28,30 before bumps 28,30 are contacted to substratebond pads 32,34. Evaporation of the solvent within the B-stage polymercauses the bumps 28,30 to retain a substantially rigid shape while theflip chip is manipulated for contacting bumps 28,30 to substrate 36. TheB-stage polymer can subsequently be polymerized to form electricalinterconnections between flip chip bond pads 12,14 and substrate bondpads 32,34.

In a preferred embodiment, flip chip 11 is aligned over substrate 36 bya flip chip aligner bonder, such as model "M-8", manufactured byResearch Devices, Division of the American Optical Corporation.

EQUIVALENTS

Although preferred embodiments have been specifically described andillustrated herein, it will be appreciated that many modifications andvariations of the present invention are possible, in light of the aboveteachings, within the purview of the following claims, without departingfrom the spirit and scope of the invention. For example, while thediscussion is directed to a single flip chip on a substrate which flipchip has only one circuit and two bond pads, it is to be understood thatthe concept can be readily expanded to include a plurality of chips witha plurality of circuits and bond pads on each.

We claim:
 1. A method of forming an electrically conductiveinterconnection between a bond pad of a flip chip and a bond pad of asubstrate, comprising the steps of:a) selectively forming a firstorganic protective layer over a surface of the flip chip around the flipchip bond pad, leaving the flip chip bond pad exposed; b) forming anelectrically conductive polymerizable precursor layer on the flip chipbond pad to a level extending beyond the first organic protective layerto produce a bump; c) selectively forming a second organic protectivelayer over a surface of the substrate around the substrate bond pad,leaving the substrate bond pad exposed; and d) forming an electricalinterconnection between the bump and the substrate bond pad, therebyforming an electrical interconnection between the flip chip bond pad andthe substrate bond pad.
 2. A method of claim 1 wherein the electricalinterconnection between the bump and the substrate bond pad is formedby:a) disposing an electrically conductive adhesive on the substratebond pad; b) contacting the bump with the electrically conductiveadhesive; and c) polymerizing the electrically conductive adhesive.
 3. Amethod of claim 2 wherein the second organic protective layer isselectively formed over the surface of the substrate to a levelextending beyond the level of the substrate bond pad.
 4. The method ofclaim 3 wherein the first organic protective layer and the secondorganic protective layer includes a dielectric polymer.
 5. The method ofclaim 4 wherein the electrically conductive polymerizable precursor isscreen printed onto the flip chip bond pad.
 6. The method of claim 4wherein the electrically conductive polymerizable precursor layer isformed by stenciling the electrically polymerizable precursor onto theflip chip bond pad.
 7. The method of claim 1 wherein the first organicprotective layer defines the area for forming the electricallyconductive polymerizable precursor layer on the flip chip bond pad.
 8. Amethod of forming an electrically conductive interconnection between abond pad of a flip chip and a bond pad of a substrate comprising thesteps of:a) selectively forming a first organic protective layer over asurface of the flip chip around the flip chip bond pad, leaving the flipchip bond pad exposed; b) forming a first electrically conductivepolymerizable precursor layer on the flip chip bond pad; c) forming asecond electrically conductive polymerizable precursor layer over thefirst electrically conductive polymerizable precursor layer, the secondelectrically conductive polymerizable precursor layer and the firstlayer together forming a bump; d) selectively forming a second organicprotective layer over a surface of the substrate around the substratebond pad, leaving the substrate bond pad exposed; e) forming anelectrical interconnection between the bump and the substrate bond pad,thereby forming an electrical interconnection between the flip chip bondpad and the substrate bond pad.
 9. A method of claim 8 wherein theelectrical interconnection between the bump and the substrate bond padis formed by:a) disposing an electrically conductive adhesive on thesubstrate bond pad; b) contacting the bump with the electricallyconductive adhesive; and c) polymerizing the electrically conductiveadhesive.
 10. A method of claim 9 wherein the second organic protectivelayer is selectively formed over the surface of the substrate to a levelextending beyond the level of the substrate bond pad.
 11. The method ofclaim 10 wherein the first organic protective layer and the secondorganic protective layer include a dielectric polymer.
 12. The method ofclaim 11 wherein the second electrically conductive polymerizableprecursor layer is screen printed onto the first electrically conductivepolymerizable precursor layer.
 13. The method of claim 11 wherein thesecond electrically conductive polymerizable precursor layer isstenciled onto the first electrically conductive polymerizable precursorlayer.
 14. The method of claim 10 wherein the electrically conductivepolymerizable precursor layers are polymerized to form an electricallyconductive bump prior to contacting the bump to the substrate bond pad.15. The method of claim 14 wherein the electrically conductive adhesiveis stenciled onto the substrate bond pad.
 16. The method of claim 8wherein the first organic protective layer defines the area for formingthe electrically conductive polymerizable precursor layer on the flipchip bond pad.
 17. A method of forming an electrically conductiveinterconnection between a bond pad of a flip chip and a bond pad of asubstrate comprising the steps of:a) selectively forming a first organicprotective layer over a surface of the flip chip around the flip chipbond pad, leaving the flip chip bond pad exposed; b) forming a firstelectrically conductive polymerizable precursor layer on the flip chipbond pad; c) drying the first electrically conductive polymerizableprecursor layer; d) forming a second electrically conductivepolymerizable precursor layer over the first electrically conductivepolymerizable precursor layer to a level extending beyond the firstorganic protective layer; e) drying the second electrically conductivepolymerizable precursor layer to form a bump on the flip chip; f)selectively forming a second organic protective layer over a surface ofthe substrate around the substrate bond pad to a level extending beyondthe level of the substrate bond pad, leaving the substrate bond padexposed; g) contacting the bump to the substrate bond pad; and h) whileso contacted, polymerizing the bump to form an electrically conductiveinterconnection between the flip chip bond pad and the substrate bondpad.
 18. A method of forming an electrically conductive interconnectionbetween a bond pad of a flip chip and a bond pad of a substratecomprising the steps of:a) selectively forming a first organicprotective layer over a surface of the flip chip around the flip chipbond pad, leaving the flip chip bond pad exposed; b) forming a firstelectrically conductive polymerizable precursor layer on the flip chipbond pad; c) polymerizing the first electrically conductivepolymerizable precursor layer to form a first electrically conductivepolymer layer; d) forming a second electrically conductive polymer layeron the first electrically conductive polymer layer to a level extendingbeyond the first organic protective layer; e) polymerizing the secondelectrically conductive polymerizable precursor layer to form a secondelectrically conductive polymer layer, the first electrically conductivepolymer layer and the second electrically conductive polymer layertogether forming an electrically conductive bump on the flip chip bondpad; f) selectively forming a second organic protective layer on asurface of the substrate to a level extending beyond the level of thesubstrate bond pad, leaving the substrate bond pad exposed; g) applyingan electrically conductive adhesive to the substrate bond pad; and h)contacting the electrically conductive adhesive to the electricallyconductive bump to form an electrically conductive interconnectionbetween the flip chip bond pad and the substrate bond pad.
 19. Themethod of claim 18 further including the step of polymerizing theelectrically conductive adhesive while the electrically conductiveadhesive is in contact with the substrate bond pad and the flip chipbond pad.